Rogue traders come in every guise and you will note that the people who get prosecuted are normally sole traders or small businesses. A recent prosecution where a young lady was killed I got involved in a discussion with the electrical safety council and my final comment to them was simple: "there but for the grace of god go I, to prison". I installed a new radial, fully tested and powered up and left. Went back a week later and plasterers had been in and taken off the sockets to plaster and not reconnected earth between sockets 2 and 3 so the result was no earth on 2 sockets (this was RCD protected) but I am willing to bet it would have been my fault and some expert witness from NICEIC or the ECA would have told them so. God only knows why I stuck my socket tester in each socket whilst I was there, I shouldn't have needed to.
A major company installed storage heaters in a house, 2 rate meter through 4 pole isolator to 2 consumer units, house and heating (timed). No problem apart from the only paperwork said "installed consumer unit, 3 storage heaters". There was no electrical installation certificate and no letter from building control, new circuits are notifiable and guess what - nobody wants to know........ Who paid the Governmant under one of these grants.....
A meter company installed a new meter whilst I was at a house and did no tests, guess what - no earth whatsoever. The metal sheath had been damaged on the TNS under the ground, enw came in and converted to TNC-s as the damage was on their damaged cable.